The film coating of solid forms such as pharmaceutical tablets, gum and confectionery pieces is increasing rapidly over other methods, such as sugar coating. Film coating has many advantages over other coating techniques, including greater efficiency, better control, and better uniformity and reproducibility of color. Endicott et al., U.S. Pat. No. 2,881,085, issued Apr. 7, 1959, and Endicott et al., U.S. Pat. No. 2,954,323, issued Sept. 27, 1960, disclose thin film coating for tablets and the like.
One of the technical problems existing for some time in the art of film coating is that of producing a color coating which may be reproduced reliably over and over again so that all batches of tablets are colored the same. One of the difficulties arises from the limited solubility of standard food color dyes in the coating solution, which results in an inability to produce fine suspensions of insoluble pigments and causes rough coatings and mottled or uneven color finishes.
Another problem arises from the fact that water is detrimental to the stability of a solid form such as a pharmaceutical tablet. Accordingly, anhydrous organic solvents are desirable as the solvent for the coloring material, but most dyes are insoluble in such solvents.
Another problem is presented by the desire to produce a smooth elegant finish on the tablets, which requires a low concentration of polymer or film-former in the coating suspension, and also a low concentration of pigments in large volumes of volatile solvents. Because of the low concentration of pigments and polymers in the solvents, producing a fine dispersion of the pigment particles in the polymer has been very difficult. In one prior technique (Endicott et al., U.S. Pat. No. 2,954,323, column 6, lines 18-19, for example), the coating suspension was ground in large ball mills for lengthy periods of time, in the order of 8 to 16 hours. This is a very time-consuming and expensive technique.